With New York City’s subway trains jammed to capacity and more people than ever pouring into neighborhoods outside Manhattan, Mayor Bill de Blasio is embarking on an ambitious and expensive plan to create a fleet of city-owned ferryboats that would crisscross the surrounding waterways and connect all five boroughs.

At a cost of more than $325 million, Mr. de Blasio’s expansion of ferry service would be one of the biggest bets any city in the world has made on boats as vehicles for mass transit. The mayor predicts that the ferries would carry 4.5 million passengers a year, about twice as many riders as San Francisco’s ferry system handles.

Mr. de Blasio has promised New Yorkers that ferries will start running on three new routes, serving South Brooklyn, and Astoria and the Rockaways in Queens, by the end of June 2017, four months before he would stand for re-election. Additional routes to the Lower East Side of Manhattan and to Soundview in the Bronx will be added in 2018.

“Our aim is to make this thing as big as possible,” said Alicia Glen, the city’s deputy mayor for housing and economic development. “No guts, no glory.”

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Mayor Bill de Blasio in March announcing that Hornblower Cruises and Events, a San Francisco-based company, will operate the ferry service.CreditUli Seit for The New York Times

Simply put, city officials believe that if New York is to continue thriving, it must have a robust transportation network and that ferries can play a critical role, just as they do in many waterfront cities around the globe.

“We’re still living with the footprint of an early-19th-century transit map that didn’t contemplate the kind of job growth we’re seeing along the waterfront,” Ms. Glen said. The administration, she said, is trying to create a transportation network for “the new New York.”

The city has already spent $6 million on four commuter boats in 2016 and could own more than 30 in a few years. Mr. de Blasio also plans to spend at least $85 million to create 13 additional landings for the ferries and a home port for them at the Brooklyn Navy Yard.

But the mayor has raised the stakes in ways few other places have by pledging that a ferry ride would cost the same as subway fare, $2.75. That is a departure from San Francisco; Sydney, Australia; and other cities where extensive commuter-ferry systems have long operated. They tend to charge more to ride ferries than buses or trains, and their ferry fares are based on the length of the trip.

The one-fare plan fits with the liberal agenda of Mr. de Blasio, who has championed “transit equity” for all New Yorkers. To fulfill the mayor’s promise, the city will have to contribute a substantial operating subsidy, a commitment that several of his predecessors were unwilling to make.

Mr. de Blasio’s former rival for the mayor’s job, Christine C. Quinn, applauded his embrace of ferries as a form of mass transit. “There’s a little bit of a whimsical, historic notion of ferries; they seem to be a lot more fun than other modes of transportation,” said Ms. Quinn, the former City Council speaker. “You don’t want ferries to just be the fun, fancy transport of people with money.”

Of course, New York’s waters were once clogged with ferries. In the early 1900s, when there were few bridges and no car tunnels, as many as 147 boats carried people across the Hudson and East Rivers.

The only vestige of that era is the Staten Island Ferry, nine hulking boats that make regularly scheduled point-to-point crossings of New York Harbor. For routes from Brooklyn and Queens, city officials have largely relied on private companies operating their own ferries to deliver workers to Manhattan every weekday.

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A design for a proposed Hornblower ferry.CreditIncat Crowther

City officials have been leaning on Hornblower Cruises and Events, the San Francisco-based company they chose in March to operate the service, to order the boats it will need. Hornblower, which runs cruises to the Statue of Liberty, has settled on a design for 149-passenger boats and is negotiating with a few boatyards around the country to build 18 of them, at a cost of nearly $4 million each.

“One of the challenges is to stand up a new fleet,” said Terry MacRae, Hornblower’s chief executive. “But it’s better than bringing a bunch of widows and orphans together,” he said, alluding to the alternative of rounding up a group of used boats.

Cameron Clark, who is overseeing the start of the ferry service for Hornblower, said the 85-foot boats were designed by Incat Crowther, an Australian company, to be fuel-efficient and spacious. The first of them are scheduled to be completed early next year, he said.

“They will have all the 21st-century stuff,” Mr. Clark said, including Wi-Fi and power outlets for laptop computers.

Maria Torres-Springer, the president of the city’s Economic Development Corporation, said Hornblower was chosen primarily for its experience in starting ferry services around the country, as well as on the Canadian side of Niagara Falls. The company, however, has limited experience with helping commuters get to and from work every day, though city officials said that did not weigh heavily against it.

Billybey Ferry, a part of the New Jersey-based New York Waterway, has been operating the subsidized East River Ferry service for the city since 2011. That service will be integrated into the citywide system after this year and will be operated by Hornblower at a reduced fare equal to a MetroCard swipe, city officials said.

Paul Goodman, the chief executive of Billybey, said his company was “disappointed to lose the bid.” But, he added, “We’re still big believers in the expansion of ferry service and we hope that it’s a success.”

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Passengers packed into the L train station at Union Square in April. City officials hope that expanded ferry service will relieve the overburdened subway system.CreditSam Hodgson for The New York Times

Mr. de Blasio announced that the home port for the expanded service would be a pier in the Brooklyn Navy Yard. But that pier is so dilapidated that it may not be rebuilt before 2018, Ms. Glen said. If the city-owned service starts next summer, as scheduled, the home port is likely to be in New Jersey at first, Ms. Glen said. The city’s ferry system, however, will not serve New Jersey.

“Homeporting is a terrific benefit of the system,” Ms. Glen said, adding that it would create jobs in Brooklyn and save on fuel costs. “If that takes another nine months, that’s not the priority.”

Hornblower will need nine boats to cover the three new routes, none of which it has now.

Mike Anderson, former chief executive of Washington State Ferries, which runs a large fleet of ferries in the Seattle area, said that to have that many boats built would normally take a few years. But Hornblower hopes to cut that schedule to one year by using three or more shipyards, including two on the Gulf Coast, Mr. Clark said.

“That’s a bit of a heavy lift,” said Mr. Anderson, an executive with KPFF Consulting Engineers who consulted with New York City on its plan.“Everything has to go right and they need to get started pretty soon.”

City officials have made provisions for delays in the production of new boats, allowing Hornblower to charter additional boats to get the service started.

The city estimates that it will cost about $70 million to have 18 ferries built. Once they are done, the city plans to buy them from Hornblower, which will operate them for six years, with a possibility of renewing the contract for an additional five years.

Ms. Glen said the city was employing “good, smart economics” in deciding to own the boats. “If, for some reason, Hornblower doesn’t perform,” she said, the city would either find another operator or run the system itself, as it does for the Staten Island Ferry. And, she added, “even if the service weren’t to be that successful, the city will have hard assets” that it could sell to recoup some of its investment.

Before the service begins, Ms. Torres-Springer hopes to find one or more sponsors for it similar to the Citi Bike bike-sharing program. But, she added, it would be premature to call the ferries Citi Boats.

Correction:

A map on Thursday with an article about a proposed expansion of ferry service in New York City omitted one of the proposed ferry terminals. One is planned for Roosevelt Island. The map also misidentified two proposed terminals in Manhattan and failed to label an existing one. The third stop from the bottom of the map, a planned stop, is Grand Avenue, not Stuyvesant Cove; the planned stop above that is Stuyvesant Cove, not East 34th Street; and the existing stop above that is East 34th Street. A corrected map can be found at nytimes.com/nyregion.

A version of this article appears in print on , on Page A1 of the New York edition with the headline: New York’s Ferry Push: Rides to 5 Boroughs, at a Subway Price. Order Reprints | Today’s Paper | Subscribe